Inner Will has a place for Disc - it is efficient to use it before you are going to pre-cast a ton of PW:Shields, then switch back to Inner Fire. It is not a good choice for anything else as Disc, and it is never a good choice as Holy, mostly because Renew, Holy's spammable Insta-cast, is so terrible (and conversely, because Disc's instant-cast, Shields, are so powerful).

I think a lot of people (my guild's resident Disc Priest included) got overly enamored with new mechanics in Cataclysm and immediately started using them without really examining them. There are a lot of abilities in WOW. A ton of them, actually. Blizzard is rapidly reaching a point in WOW where they are running out of abilities to give classes (if you assume that Blizzard cares about class distinctions and won't just give every class every ability in the game eventually). I believe Inner Will is a symptom of this. When I look at IW, I see a bunch of developers sitting around a table thinking along these lines:

"Priests are tired of the counters on Inner Fire."
"But what's the point of the spell if they don't have to make any choices or pay attention to it? What about Warlocks and Mages, they have to switch armor spells sometimes...kinda..."
"We're having a hard time coming up with another ability for Priests since they have so many spells already."
"Ok, lets give them an armor spell. It will use up one of the three ability slots for Priests in Cataclysm. Who knows, some Priests might even actually use it."

Blizzard is rapidly reaching a point in WOW where they are running out of abilities to give classes (if you assume that Blizzard cares about class distinctions and won't just give every class every ability in the game eventually).

I actually love the variety of abilities in our kit, not all priests do everything exactly the same and I'm a fan of the personal touch.
I'm not a fan however of the number of binds I'm sporting this expansion. I got away with not using a Naga Razor mouse this time but next expac I'll definitely need one. t.t

Inner Will has a place for Disc - it is efficient to use it before you are going to pre-cast a ton of PW:Shields, then switch back to Inner Fire. It is not a good choice for anything else as Disc, and it is never a good choice as Holy, mostly because Renew, Holy's spammable Insta-cast, is so terrible (and conversely, because Disc's instant-cast, Shields, are so powerful).

I think a lot of people (my guild's resident Disc Priest included) got overly enamored with new mechanics in Cataclysm and immediately started using them without really examining them. There are a lot of abilities in WOW. A ton of them, actually. Blizzard is rapidly reaching a point in WOW where they are running out of abilities to give classes (if you assume that Blizzard cares about class distinctions and won't just give every class every ability in the game eventually). I believe Inner Will is a symptom of this. When I look at IW, I see a bunch of developers sitting around a table thinking along these lines:

"Priests are tired of the counters on Inner Fire."
"But what's the point of the spell if they don't have to make any choices or pay attention to it? What about Warlocks and Mages, they have to switch armor spells sometimes...kinda..."
"We're having a hard time coming up with another ability for Priests since they have so many spells already."
"Ok, lets give them an armor spell. It will use up one of the three ability slots for Priests in Cataclysm. Who knows, some Priests might even actually use it."

I definitely found Inner Will extremely useful while in mostly 333/346 blues. I guess I never really thought to stop using it until just recently.

I've been using Fire since about two weeks into Firelands. My regen is high enough where unless it's a burn phase where I'm spamming the daylights out of PoH, I'm golden. Even then, I can usually get by okay, and Will won't make that huge of a difference in the grand scheme of things.

Don't equate "I am constantly running out of mana" with "Inner Will is helping." If you aren't casting that many instants, it really isn't helping all that much.

^ This exactly. There are only a few random situations where Inner Will might be an option (as was mentioned previously), and then you immediately swap back to Inner Fire. When I first saw the spell I thought, lose 532 spell power (which affects ALL of my spells) for a miniscule amount of mana cost reduction on ONLY instant casts? No way. If you find yourself running out of mana, then you need to evaluate why. Are you using too many inefficient heals? Were people just being dumb and taking unnecessary damage? Am I trying to keep up an undergeared tank with no mitigation? Every time I see a priest using Inner Will full time when I'm running my alts, it makes me sad.

Inner Will does have its place, especially on Heroic Rag. You do shield quite frequently, and its a noticeable difference for me as disc. I do stay in will for the duration of the fight. Overall though, for normal modes and 6/7HM you should use Inner Fire. Just my 2c from personal experience. As for Holy, I would say Inner Fire no contest.

Yeah, actually I did. I made two spreadsheets, one for each spec. If you are Disc, with a heavy Mastery setup, and you are spamming PW:S before a big hit goes out (so that most or all of their absorbing power is used up) it makes sense to use IW. There is no other time when it makes sense to use IW as Disc, and it never makes sense with the math as Holy.

Don't equate "using IW" with "casting more instants"

This statement makes no sense. If you are casting lots of instants, IW makes sense, but if you aren't, you're just wasting 500 spellpower (which is about 2 pieces of gear's worth at 378 ilvl btw, not a small amount at all). Maybe you think IW applies to all spells?

Do you offer anything to back this up? Either actual use of IW or some math?

I can break out the math on the IW for Disc statement I made above if you like, but otherwise I think it just doesn't pass the common sense test. Since IW only affects instant casts, wouldn't it make sense to not use IW if you aren't using instant casts? I don't need a spreadsheet to tell me the answer to that one.

Do you offer anything to back this up? Either actual use of IW or some math?

The math you need to solve IW vs IF is not actually called Math but called Logic

IW - Mana Savings, no one needs mana nowadays, that doesn't mean you can spam heals, you need to be smart about what you cast - IW generally means you can cast 1 more spell for each 10 you cast... that makes for 10 free casts in a long long fight, just doesn't worth it

IF - Increases ALL your healing done, most fights nowadays are about throughput and not mana management

Even in the fights where you might go OOM it's not because you're casting too many instants, it generally the fights where you need to cast more (that means PoH, because you won't solve big healing surges with renews)

No priest has been using IW regularly since they finished HC's, the only situation where IW comes ahead is when you're running and have a spare GCD to change "stances" and it happens it's needed to keep on the move - you can fire your renews, CoH and PoMs then (all this ofc it's not intensive healing phase otherwise you just need IF on and say: "Fuck you mana"